Studies in the dynamics of disinfection: XII. The effect of variation in pH on the rate of disinfection at 51°C. of standard cultures of Bact. coli.

Abstract

1. Standard Bact. coli cultures have been heated at 51° C. in phosphate buffered media having pH values ranging from 2·8 to 8·8, and survivor curves obtained.2. Except at pH 3·9, the first continuous decline in viable numbers was followed by the establishment of a small fluctuating residual population which was apparently permanent.3. The colony counts showed a moderate excess of unduly high values of x2, the bulk of which were encountered when the mortality was between 95 and 99·999%.4. Prolonged incubation of the plates led to increased colony counts, but the rise was confined to the active disinfection phase. It occurred chiefly in the 95–99·999% mortality range and was moderate in degree. The x2 values were little affected.5. The log survivors-time graphs of the disinfection phase were convex in type but approached linearity in the faster reactions. The logarithmic death-rate became maximal at about 90 % mortality and thereafter remained constant, though under the more acid conditions a somewhat lower rate was maintained. At pH 7·0 and 6·4 the curves were exceptionally straight.6. Standardization of all the curves revealed a continuous change in shape with pH but, nevertheless, they could be placed in three closely knit overlapping groups which were best revealed by the variation in maximum death-rate.7. The 50–99·99% mortality times were obtained either from the calculated regressions of log survivors on time over the linear portions of the curves or, outside that range, from the freehand curves. Maximum survival at all mortality levels occurred at about pH 6·5–6·6, i.e. close to the neutral point of pure water at 51° C. On both sides of this point sensitivity increased regularly but was abnormally high at pH 7·0 and 6·4.8. Within these zones of abnormally high sensitivity the assessment of the effect of a given change in pH varied in amount and direction according to the mortality level chosen.9. The enhanced resistance of a culture regenerated from the survivors of a disinfection at 51° C. and pH 7.0, as previously reported, could possibly have been largely due to a small increase in acidity but not to a slight shift towards alkalinity.